There is increasing interest in producing fuel ethanol from lignocellulosic feedstocks such as, for example, wheat straw, corn stover, and switch grass.
One of the primary processes for producing ethanol from lignocellulosic feedstocks is to carry out a pretreatment, followed by enzymatic hydrolysis of the cellulose to glucose. The pretreatment is often carried out by exposing the feedstock to dilute sulfuric acid and high pressure steam for a short period of time. One process for doing this is steam explosion (generally disclosed in U.S. Pat. Nos. 4,461,648 and 5,916,780). Steam explosion pretreatment greatly improves the subsequent enzymatic hydrolysis of the cellulose.
Steam explosion pretreatment can be carried out in a batch or a continuous manner. Continuous operation is preferred because the productivity of the pretreatment reactor is greater than a batch reactor due to the time required to fill and empty batch reactors. In addition, there is a limit to the size at which a batch steam explosion reactor can uniformly pretreat the material. This limitation on size results in a requirement for a large number of batch steam explosion reactors in a commercial size ethanol plant.
On the other hand, a continuously-operating steam-explosion pretreatment achieves a high productivity with a good enzymatic hydrolysis of the cellulose. However, the lignocellulosic feedstock must be conveyed continuously into and through the pretreatment reactor. This may be achieved by preparing a slurry consisting of finely chopped lignocellulosic feedstock in water. The addition of water facilitates the transportation and mechanical handling of the lignocellulosic feedstock in unit operations upstream of and within the pretreatment reactor. Typically, the mass of water present is usually at least 5 to 25 times the mass of feedstock solids present for the slurry to flow uniformly.
In a conventional method of the prior art, the slurry of lignocellulosic feedstock is pressurized above the pressure in the pretreatment reactor using a series of specially engineered pumps and such slurry of lignocellulosic feedstock is heated to the reaction temperature prior to its introduction into the pretreatment reactor. This heat-up is accomplished by the injection of high pressure steam at elevated temperature. The amount of steam and acid needed for this heat-up is a direct function of the total mass of the slurry, including the water addition for transportation of the slurry. Thus, the presence of a large amount of water requires a large amount of steam for the heat-up as well as a large amount of acid. In the pretreatment reactor, the slurry is maintained at an elevated temperature for a predetermined length of time. After the pretreatment reaction is complete, the slurry of pretreated lignocellulosic feedstock is cooled by discharging it through a series of flash vessels wherein a significant amount of the original steam added can potentially be recovered as flash steam at substantially lower pressure and reused to preheat the incoming lignocellulosic feedstock slurry. Steam that could not be recovered remains as condensate in the slurry and is a source of additional dilution of the slurry.
During the downstream processing of the lignocellulosic feedstock that follows the pretreatment, substantially all the added water is removed at a significant cost by, primarily, evaporation or distillation processes. Thus, the addition of water for transportation of feedstock contributes to large steam usage, a large amount of acid usage as well as large evaporation or distillation systems that add significant capital and operating costs of the ethanol production process.
U.S. Pat. No. 4,842,877 discloses a process in which a biomass substrate is first prehydrolyzed with a reaction medium containing strong alkali and then further treated with a chelating agent to remove metal ions, thereby avoiding the formation of unwanted precipitates on process equipment. The biomass product of the chelating step is subsequently fed to a pressurized extruder reactor into which hydrogen peroxide is added, along with oxygen. The oxygen serves to activate the hydrogen peroxide while the effect of friction and pressure in the extruder accelerates the reaction of the biomass with the hydrogen peroxide. The extruder apparatus is preferably a Wenger TX-138, X-175, X-185 or X-200 continuous extrusion cooker.
U.S. Pat. No. 4,427,453 discloses an apparatus in the form of a worm feeder which consists of a conical, pressure-resistant housing having a radial charging opening at its larger diameter end and a cylindrically-shaped, axial, exit sleeve at its smaller end. The material is injected into the charging opening and is moved by a rotating conical worm under strong compression, and thus high pressure, to the smaller end where it is forced through the exit sleeve as a compressed plug. The conical housing is provided with perforations so liquid is squeezed out from the material during the compression.
U.S. Pat. No. 6,251,643 discloses a screw press having chambers for carrying out of stages of pressing and treating aqueous suspensions of material. The chambers are axially disposed in line with at least one common integral shaft having screws to convey the material. During the operation of the screw press, a plug of compacted material is formed at the exit end of each screw. This plug seals off one chamber from the next chamber.
U.S. Pat. No. 7,347,140 discloses a screw press for separation of liquid from solid/liquid mixtures, the screw press having a casing with perforations for liquid withdrawal. The casing includes a screw shaft with a circular gap through which the liquid is pressed. A counter pressure device creates a backup of the solid/liquid mixture so as to increase the pressure in the circular gap to extract more liquid from the solid/liquid mixture.
WO 96/25553 discloses a lignocellulosic dewatering system operating at atmospheric pressure to press the lignocellulosic material into an insert, which is in the form of a compact plug, whose purpose is to separate the atmospheric medium of the press from the high pressure medium in a hydrolyser.
A final report titled “Second Stage Countercurrent Reactor” prepared for the National Renewable Energy Laboratory, Golden Colo., dated Oct. 19, 2000 and published by the Harris Group discloses a first stage horizontal reactor operating at 150 psig and 185° C. and a second stage vertical reactor (also referred to as “digesters”) operating at 370 psig and 225° C. An atmospheric plug screw feeder compresses pre-steamed chips to form a tight plug prior to entering the first stage, and a pressurized Tee-Pipe Assembly that provides a pressure seal for the first stage horizontal digester. In the second stage hydrolysis, a pressurized plug screw feeder compresses the pressurized, partially cooked chips from the first stage reactor to form a tight plug against the reactor pressure. A pressurized Tee-Pipe Assembly provides a pressure seal for the second-stage counter-current reactor. A pressurized shredder conveyor breaks up the plug and continuously feeds the second-stage vertical digester where the partially cooked chips from the shredder are hydrolyzed with acid.